Because cured articles prepared from an epoxy resin have excellent adhesion, mechanical properties, thermal properties, chemical resistance and excellent electrical properties, they are utilized widely and commercially as paints, adhesives, and electrical and electronic insulation materials. Epoxy resin formulations used for such applications can be either a one-component system or a two-component system.
The two-component system is made of an epoxy resin composition and a curing agent which are separately stored. When needed, they are weighed and mixed just prior to use. In so doing, it is often difficult to avoid mistakes in weighing the correct amount of epoxy and curing agent, as well as difficult to form a homogeneous curing composition. These mistakes are further compounded by the fact the reaction of the epoxy resin and the curing agent starts as they are mixed.
The epoxy resin/curing agent composition is typically in liquid form. The viscosity of the system increases gradually, goes through gelation, and finally the system cures. The time from formulation until it can no longer be used is called "pot life". The pot life is decided by the chemical structures and the ratio of the epoxy resin and the curing agent employed. Generally, the system with a faster curing rate has a shorter pot life. If a curing agent which is designed with an emphasis on the curing rate is used, a formula can usually be cured at room temperature or lower. However, it will naturally shorten the pot life and it creates a need to frequently formulate a small amount of the composition. Thus, such room temperature curing compositions cause a reduction in working efficiency.
In contrast, because a curing agent is added ahead of time in the epoxy resin of the one-component system, the above-mentioned problems associated with the two-component system can be eliminated. The curing agent used in such systems is typically called a "latent curing agent". The simplest one-component system is based on the curing agent which can cure epoxy resin at high temperatures. Such agents include, but are not limited to, dicyandiamide, phenol novolak, adipic dihydrazide, diallyl melamine, diamino maleonitrile, BF.sub.3 -amine complexes, amine salts, and modified imidazole compounds and the like. Because these curing agents require a high temperature a slow curing rate is inherent. Thus, the reaction proceeds slowly at room temperature and the epoxy resin compound formulated with curing agents can be stably stored at room temperature or lower temperatures for a certain period. Thus, these systems are especially suitable for one-component systems which are cured at high temperatures.
Such high temperature curing agents do not dissolve in an epoxy resin at room temperature. If such a curing agent is dispersed as particles, the storage stability can be improved drastically. This is due to the small contact area with the epoxy resin. In this case, a small particle size for the curing agent is important because smaller particle sizes increase the curing rate and also make the structure of the cured article very uniform. See J. Appl. Polymer Sci, 32, 5095(1986) Such a dispersion type curing agent can also be called a latent curing agent.
Generally, the epoxy resin curing composition of a one-component system, in its normal state, requires a latent curing agent which can be activated by some form of stimulation. Examples of such latent curing agents include amineimide compounds which are activated by thermal decomposition; ketimine compounds which are activated by contact with moisture; aromatic diazonium salt compound, diallyliodonium salt compounds which are activated by exposure to light; and curing agents which are microencapsulated in material which can be destroyed by mechanical pressure or by heat. However, due to problems such as performance and cost, they are not widely used. As a link before they become truly useful, currently the above-described latent curing agent of the dispersion type is important because the preparation method involved is simpler and less expensive.
However, a particularly promising latent curing agent is the modified amine curing agent adduct obtained by reaction with an epoxy compound. Due to the modification by the epoxy compound, the flaws of amine curing, such as volatility which causes problems in handling, hygroscopicity which significantly affects curability, and compatibility with the epoxy resin, can be improved. Even further, control of the melting point is also possible. Epoxy resin is cured by polyaddition reaction with a curing agent or by ionic polymerization.
However, because curing agents which require secondary processing tend to be expensive, a curing agent of a catalytic polymerization type which does not depend on the addition of equivalent amounts and can be cured with the addition of a small amount, will be an advantage. From the aspect of performance, the curing agent of the anionic polymerization type which does not risk corrosion of metal (tertiary amine adduct) is preferred. A suitable catalyst for this purpose is an imidazole/epoxy resin adduct, and its technology is disclosed in detail in Japanese Patent Publication (Kokai) No. SHO 58-13623 (1983) and SHO 61-268721 (1986). This solid adduct which is synthesized from amine compounds and epoxy resins can be obtained by reacting an amine compound and an epoxy resin in a solvent and then removing the solvent from the system as a whole. Then, it is crushed and pulverized and then sieved to obtain particles of the curing agent of a desired size. There is a limitation to the degree of crushing and pulverization, and it is extremely difficult to commercially prepare fine particles having a Stokes diameter of smaller than about 4 .mu.m.
Because the above-described preparation method is a long and laborious process, it incurs a very high production cost. In addition, due to the limitation of the particle size of the curing agent formed by crushing and pulverization and also to the pulverized state, there are the following disadvantages:
a. bulky and inconvenient for packaging and shipment, PA1 b. aggregation of the particles require enormous work to disperse the particles in an epoxy resin when used, PA1 c. increase in viscosity takes place when added to an epoxy resin, PA1 d. a limit in the degree of improvement on the curing rate by reduction in particle size, and PA1 e. relatively short stable storage period of the formulated curing composition. PA1 (a) dispersing spherical particles of an amine compound/epoxy compound adduct having a melting point of at least 50.degree. C. and a diameter of 0.1 .mu.m to 30 .mu.m in a liquid epoxy resin; PA1 (b) adding 1 to 100 parts by weight of a polyfunctional isocyanate compound based on 100 parts by weight of a polyfunctional isocyanate compound based on 100 parts by weight of said spherical particles at a temperature below the melting point of said spherical particles with agitation; and PA1 (c) heating and agitating the resulting mixture at said temperature until the isocyanate groups in said polyfunctional isocyanate compound have been completely reacted.
Although the particles of the amine compound/epoxy compound adduct has a variety of advantageous features as a curing agent, such advantages have not been used to the full extent with the one component curing composition due to the reasons mentioned above.
On the other hand, the curing agents which are more widely used are ones obtained by treating solid particles of an amine compound/epoxy compound adduct with a polyfunctional isocyanate to improve its latent curability. See Japanese Patent Publication SHO 64-70523 (1989) and HEI 1-113480 (1989). In this case it is presumed that the polyfunctional isocyanate has been reacted with the particles of the adduct to form an encapsulated film on their surfaces. With this type of latent curing agent which tends to be expensive, it will be advantageous to use an ionic polymerization type curing agent which can be cured with a small amount of addition without regard to the addition of equivalent amounts. From the performance viewpoint, anionic polymerization type curing agents (tertiary amine adducts) which have no risk of corrosion of metal are preferred. The amine compound/epoxy compound adduct can be obtained firstly as a lump by removing the solvent from the reaction system after reacting the amine compound and the epoxy resin in a solvent. Subsequently, it is crushed and pulverized, and then classified and a curing agent of a desired size is removed. Subsequently, the particles of the amine compound/epoxy compound adduct are dispersed in a liquid epoxy resin, and a polyfunctional isocyanate compound is added and reacted in a heated state to prepare a desired latent curing agent as a masterbatch. In Japanese Patent Publication (Kokai) No. HEI 1-113480 (1989), it is assumed that the polyfunctional isocyanate compound added has been adsorbed on the particles of the amine compound/epoxy compound adduct dispersed in an epoxy resin, and that the hydroxyl group therein and the moisture therein are reacted to form a polyurethane polymer and a polyurea polymer to form an encapsulating film. The latency of the agent is imparted by this thermal fusible film which prevents direct contact of the particles of the adduct with the epoxy resin. Further, the polymer composition of the encapsulating film is controlled by the amount of the moisture contained in the particles of the adduct to allow the production of the particles of an encapsulated adduct which has enough solvent resistance against an organic solvent added only in the presence of a suitable amount of water and can withstand the mechanical mixing treatment in the compounding step.
The particles of the amine compound/epoxy compound adduct used for the production of the above described prior art curing agent masterbatch are in the form of relatively large pulverized particles having an average Stokes diameter of about 3 .mu.m or more. Due to the shape of these particles, there are created various following adverse problems when used as the curing agent masterbatch.
The increase in viscosity is greater when using pulverized particles than when using spherical particles. Accordingly, in regards to a curing agent masterbatch, since there is a limit as to the viscosity which can be handled, the concentration of the curing agent has to be lowered in a relative manner. Therefore, when this curing agent masterbatch is added to prepare an epoxy resin curing composition, no problem emerges if the epoxy resin to be cured is the same epoxy resin as the dispersing medium of the curing agent masterbatch. However, if they are different, the desired properties of epoxy resin to be cured will be diluted by the epoxy used as the dispersing medium. Further, it is not desirable for the viscosity to increase during formulation since it will lower the degree of freedom of formulations design. For example, in order to lower the thermal expansion coefficient of the epoxy resin, a filler is usually added to the epoxy resin curing composition. However, the addition of the filler also increases the viscosity of the composition and accordingly, it is preferred to use a composition having a low as possible viscosity in order to increase the amount of the filler which can be added. For this reason, even if the curing agent of an amine compound/epoxy compound adduct treated with a polyfunctional isocyanate compound has various advantages, such advantages are not fully used in the one-component curing composition.